Some were classified files that Edward Snowden, a former N.S.A. contractor, gave to reporters; some have been in the public record for years; some were recently declassified by the government; and others are judicial opinions released by judges now sorting through the constitutional issues raised by the revelations about the program. As we begin the New Year, here’s a short history of metadata collection and the Obama Administration’s response to it, as told by an assortment of the most important documents.

1. December, 1999: Internal warnings about “electronic surveillance.”

In 1999, according to an N.S.A. Inspector General report leaked by Edward Snowden, lawyers at the Clinton Justice Department ruled that searching telephone metadata amounted to unauthorized surveillance of Americans:

NSA proposed that it would perform contact chaining on metadata it had collected. Analysts would chain through masked U.S. telephone numbers to discover foreign connections to those numbers, without specifying, even for analysts, the U.S. number involved. In December 1999, the Department of Justice (DoJ), Office of intelligence Policy Review (OIPR) told NSA that the proposal fell within one of the FISA definitions of electronic surveillance and, therefore, was not permissible when applied to metadata associated with presumed U.S. persons (i.e., U.S. telephone numbers not approved for targeting by the FISC).